The measurements of filtering parameters under breathing and feeding of filter-feeding silver carp (Hypophthalmichthys molitrix Val.)

As a typical filter-feeding fish, breathing process of silver carp is combined with its feeding process when it filters plankton in water. The present experiments studied the relationships between breathing and feeding of silver carp in response to the reduction of dissolved oxygen (DO) levels. The results showed that (1) in the clean water without phytoplankton, normal DO levels (5.43–7.73 mg/L) did not affect significantly the respiration of silver carp in terms of respiratory frequency (fR), oxygen consumption rate (VO2), respiratory stroke volume (VS.R), gill ventilation (VG) and oxygen extraction efficiency (EO2); while DO level declined to 4.40 mg/L fR, VS.R and VG increased significantly and EO2 decreases significantly; The VO2 of silver carp reached the peak when DO levels declined to 2.21 mg/L; The critical oxygen level of silver carp was about 1 mg/L, at which the EO2 of fish was substantially decreased to the lowest value. (2) fR, VS.R, VG and VO2 of silver carp under feeding condition were significantly higher than those in clean water at oxygen levels ranging from 3.37 to 7.73 mg/L; While the EO2 of the fish under feeding condition was lower significantly than that in clean water; VS.R, VG and VG/VO2 of silver carp under feeding condition increased significantly when DO levels declined to 2.21 mg/L. In summary, silver carp fed actively and respired passively when DO levels were above 3 mg/L, i.e., it showed higher VG, higher FR and stable VO2. While silver carp fed passively and respired actively when DO were below 3 mg/L, i.e., it showed sharply increased VG, decreased filtering efficiency (E) and anti-filtering response.